Newbie question (HELP!)

[RockStarWunnaB]

So I'm getting the hang of things still, and about 2 days ago I started on my very first project...a scarf. I cast on 25 stitches and about 10 rows into the scarf, I realize that I've added on about 23 extra stitches, and the edges, instead of looking square, were rounding off. Discouraged, I pulled it apart to start over...again. Any suggestions as to why this is happening?

[MMario]

something a lot of people do is accidently loop the yarn over the needle at the beginning of a row - and then knit it as a stitch later; which means you add stitches